The invention relates to a crankshaft for a reciprocating internal combustion engine, containing a crankshaft journal and an eccentric sleeve which is mounted rotatably by means of its cylindrical inner surface on the crankshaft journal.
A crankshaft of the type mentioned at the beginning is disclosed in DE 197 03 948 C1. A crankshaft of this type serves to change the compression of a reciprocating internal combustion engine by the fact that the effective length of the connecting rod is shortened or extended, as described below.
A connecting rod is fastened in a known manner by means of its first end in an articulated manner on the piston which moves up and down in the cylinder of the engine. The connecting rod is fastened by means of its second end in an articulated manner to the crankshaft journal of the crankshaft. The linear up and down movement of the piston is converted via the connecting rod into a continuous rotational movement of the crankshaft. According to DE 197 03 948 C1, the connecting rod in this case is mounted by means of its second end on the crankshaft journal via an eccentric sleeve connected in between. The eccentric sleeve has a cylindrical inner surface and a cylindrical outer surface which is eccentric with respect to the inner surface. Depending in each case on the rotation of this eccentric sleeve relative to the crankshaft journal and to the connecting rod, a different effective length results for the connecting rod, which can be defined as the distance between the axis of rotation of the connecting rod on the piston and the axis of rotation of the connecting rod on the crankshaft journal. By means of a rotation of the eccentric sleeve, the compression occurring during the displacement of the engine piston, and therefore the engine performance, can thus be changed.
One of the drawbacks of the known arrangement is that the automatic rotation of the sleeve may cease to happen over a relatively long period of operation of the reciprocating internal combustion engine due to inevitably occurring abrasion. In this case, the correct functioning of the change in length of the connecting rod is no longer ensured.
JP 57040 discloses another mechanism for changing the effective length of a connecting rod. In the case of this mechanism, the bearing journals of a crankshaft are mounted on the vehicle bodywork via eccentric sleeves connected in between. The sleeves can be rotated via an electric drive, with the result that the axial position of the crankshaft, and therefore the effective length of each connecting rod situated thereon, changes. A drawback of this arrangement is that the change in position of the entire crankshaft requires complicated adaptations in the preceding and subsequent power train. Furthermore, all of the bearings of the crankshaft have to be rotated synchronously so that the axis of the crankshaft does not become tilted. A further drawback resides in the fact that the change in length can only be undertaken for all of the connecting rods of the engine simultaneously.
Against this background, it was the object of the present invention to improve a crankshaft of the type mentioned at the beginning in such a manner that it is set up in a simple manner for reliable operation of the engine in the long term.
This object is achieved by a crankshaft having the features of claim 1. Advantageous refinements are contained in the subclaims.
Accordingly, the crankshaft for a reciprocating internal combustion engine contains at least one crankshaft journal which is arranged in a customary manner eccentrically with respect to the axis of rotation of the crankshaft. Furthermore, the crankshaft contains an eccentric sleeve which is mounted rotatably by means of its cylindrical inner surface on the crankshaft journal and the cylindrical outer surface of which lies eccentrically with respect to the inner surface. The one end of a connecting rod can be mounted on the outer surface of the sleeve, a rotation of the sleeve about the crankshaft journal enabling the effective length of the connecting rod to be changed on account of the eccentricity of the sleeve. The crankshaft is defined in that the sleeve is in engagement with a drive means in such a manner that it rotates relative to the crankshaft journal only when the drive means is activated. In this case, the drive means is preferably fastened to the crankshaft, so that it rotates together with the latter.
By connecting the sleeve to an active drive means it can be ensured that even at an advanced age of the engine and with the wear which occurs in this case, the sleeve is still moved in the desired manner in order to bring about the required changes in length of the connecting rod. Since the drive means rotates only when activated, it ensures at the same time that the sleeve is retained in the desired positions when it is rotated about the crankshaft journal, said positions corresponding as a rule to the minimum or maximum effective length of the connecting rod. In contrast, without activation the drive means is at a standstill and then also fixes the sleeve in its current position.
According to a preferred refinement, the drive means can be formed by a motor which drives a worm, the worm engaging in a toothing formed on the sleeve. In this arrangement, the motor can set the worm into a rotational movement about its longitudinal axis, said movement in turn being converted via the toothing of the sleeve into a rotational movement of the sleeve.
The gear mechanism comprising the toothing of the sleeve and the worm is preferably designed to be self-locking. This means that without an active rotation of the worm by the motor no relative movement takes place between the toothing and the worm. In particular, torques acting on the sleeve cannot lead to a rotation of the sleeve, since they are not capable of causing the rotation of the worm needed for this. Such a self-locking design of the gear mechanism has the advantage that it remains functional even if the active drive systems fail and securely retains the sleeve in its set position.
According to a development of the invention, the drive means can be designed hydraulically. This means that the necessary kinetic energy is provided via a pressurized hydraulic medium, such as a hydraulic oil.
The supply of the hydraulic medium to a hydraulically operated drive means can take place in particular via separate feed channels which can be formed within the crankshaft. However, the drive means can also be connected to the existing oil supply which is formed within a crankshaft in order to supply the bearings with lubricant. A connection of this type of the drive means to the oil supply of the crankshaft preferably takes place via at least one valve which can be opened and closed in a manner controllable from the outside. In this manner, the drive means can be activated and deactivated externally.
As an alternative and/or in addition, the drive means can also be designed in such a manner that it can be operated electrically. The drive means may, in particular, be an electric motor.
The necessary electric power is supplied to such an electric drive means preferably via lines which run through the crankshaft and are connected to sliding contacts, so that the electric current can be transferred to the rotating crankshaft from a current source connected to the vehicle body.